OF-15-11 Geology and Groundwater Resources of Park County

This open-file report is intended to provide a regional overview with the general public in mind, although it contains detailed background that may be beneficial to more technical users. This multiyear (2011-2016) project was made possible with multiple sources of funding, including grants from the Colorado Water Conservation Board (CWCB) as well as the Park County Land and Water Trust Fund (LWTF). The Colorado Geological Survey (CGS), through its Severance Tax and/or Colorado General Operational Funds, and CUSP have provided match.

Park County has experienced considerable population growth in recent decades with development becoming increasingly reliant on local groundwater resources. A diverse geologic setting characterizes the county and groundwater may be found in many of those settings. This product compiles the most recent geologic mapping and interpretations focusing on groundwater occurrences in the various geologic formations found in the area. The 2017 revision adds a discussion of groundwater types and quality, as presented in more detail below. This open-file report is intended to provide a regional overview with the general public in mind, although it contains detailed background that may be beneficial to more technical users.

Park County straddles two very different geologic terrains that share a long and complex history. The east side of the County extends into the Precambrian cored Front Range uplift of the Rocky Mountains. The west side of the County covers the South Park topographic basin, a 50 miles long by 35 miles wide structural feature shaped by a long and varied history of geologic processes. It contains a wide variety of crystalline igneous and metamorphic rocks, volcanic rocks, and sedimentary units, ranging in age from Precambrian through Cenozoic. Based on differences in hydrologic properties, aquifers and confining units in Park County can be grouped into three general categories: 1) crystalline-rock aquifers, 2) sedimentary bedrock aquifers and confining units, and 3) unconsolidated Quaternary deposits. Within these categories, there were 19 mapped hydrogeologic units.

There were 7,656 completed water wells in the Colorado Division of Water Resources database, inventoried in Park County as of December 19, 2012. Permitted uses include: domestic or household use only, livestock, commercial, industrial, municipal, irrigation, monitoring and other (such as evaporative, fire, geothermal, gravel, or unspecified). Wells and springs were assigned a hydrogeologic unit. Due to various sources of uncertainty, hydrogeologic unit designations were assigned a confidence level value of 1, 2, or 3, with 3 representing the least confidence.

A Park County groundwater quality database was compiled from publicly available databases, reports, and samples collected in 2011, 2012, 2014 and 2016 by the Coalition for the Upper South Platte (CUSP) with some assistance by CGS. The primary electronic data source was the Water Quality Portal (WQP) from the National Water Quality Monitoring Council. Groundwater data was also manually compiled from other publicly available publications not in the WQP. A total of 689 sample sites had groundwater quality data through 2016. A limited number of sample locations have data from multiple sampling events at the same well or spring. Data analysis for these locations used the maximum value detected, unless it was identified as an anomalous outlier. Where feasible, dissolved water quality data was used preferentially over total data.

Water quality in sampled Park County wells and springs is variable and dependent on local geology, geography, and seasonal influences. It is emphasized that the existing database spans a time period of multiple decades and the data do not represent a synoptic view of water quality conditions. The water quality database was evaluated for water type in each of the hydrogeologic units. Cations (sodium, potassium, calcium, and magnesium) and anions (chloride, bicarbonate, carbonate, and sulfate) data were used to generate Stiff diagrams and/or pie charts. Pie charts were scaled in size relative to the total dissolved solids (TDS) concentration, with larger pies having higher measured TDS. Additionally, groundwater quality was evaluated against the federal Environmental Protection Agency/Colorado maximum contaminant level (MCL) primary and secondary standards for drinking water. Constituents having primary and/or secondary MCL exceedances were mapped using circles of varying sizes that were blue for concentrations at or below the MCL and red if they exceeded the MCL. One or more exceedances of the primary MCLs were observed for fluoride, nitrate, arsenic, beryllium, lead, uranium, and gross alpha. Similarly, exceedances of the unenforceable secondary MCLs were noted for pH, total dissolved solids, chloride, fluoride, sulfate, aluminum, iron, manganese, and zinc. The most common constituents exceeding primary or secondary drinking water standards were manganese, sulfate, uranium, and gross alpha.